The present invention relates to illumination of an instrument for displaying information. In particular the invention relates to illumination for an instrument suitable for use in a vehicle.
Such instruments include clocks, hand held computer games, mobile phones, in fact any instrument which is capable of displaying information electronically. Instruments for displaying information on a vehicle display are usually located on the dashboard of an automotive vehicle, and require some form of illumination to allow the driver to read the information when it is dark. An instrument usually comprises an information bearing surface in a housing having a transparent cover for protection of the instrument. The information bearing surface usually has symbols indicating a scale such as graphics, numbers or a dial, and a pointer for indicating a point within the scale, or may comprise a liquid crystal display suitable for displaying numeric information. The information bearing surface may also include features such as rings defining the perimeter of a particular instrument or gauge. The instrument is generally mounted on a printed circuit board (PCB) which is connected to sensors (directly or indirectly via a network) which monitor certain engine conditions such as speed, temperature and fuel level to name but a few. Conventionally instruments are mounted together to form an instrument cluster. As customers become more sophisticated in their requirements, it becomes important to provide lighting for vehicle instruments which is both distinctive, and cost effective to produce.
In general one of two types of illumination are used, either front lighting or back lighting. Front lighting refers to illumination from the viewing side of the information bearing surface which reflects off the information bearing surface. The light need not be perpendicular to the information bearing surface, hence this term includes side lighting, where the light is reflected from the information bearing surface. The information is printed so that the light reflecting from the symbols is in contrast to the light reflecting from a background, for example the symbols may be black and the background may be white or the symbols may be white and the background may be black. When the symbols are lighter than the background the illumination is known as negative mode, and when the symbols are darker than the background the instrument is described as positive mode.
Back lighting refers to illumination of the information bearing surface from the side opposite to the viewing side of the instrument. In this case the information surface contains opaque or semi opaque portions so that the light passing through the information bearing surface renders the symbols visible.
Backlighting of vehicle instrument panels has become more common as it is currently considered to provide a better overall appearance for the vehicle instrument panel because it is not required to mount lighting components in front of the instrument. However, front lighting provides a more pleasing appearance of the instrument when illuminated.
Front or back lighting may be achieved either by providing illumination directly from a light source (which may include the use of reflective surfaces), or indirectly from a light source which is coupled to a light guide which directs the light from the light source to the required illumination point.
A problem with known instruments which have an associated light source is that, while front lighting allows a simpler design for the information bearing surface and any associated pointers any cost savings resulting from these simplified designs are negated by the extra costs required for flexible connectors from the PCB to the light source or complex light guides, and for more complex assembly. Front lighting from a point source also often produces shadows. Back lighting on the other hand usually involves the use of a plurality of light sources, and a more complex arrangement is required to illuminate the pointer. Furthermore, applying symbols to the information bearing surface is more complex in the back lighting case because poor distribution of the illumination can cause portions of the instrument to have better illumination than others. This is compensated for by providing different thickness of opaque or semi opaque portions in some areas than in others
A further problem with backlighting is that as light sources, in particular light emitting diodes (LEDs) develop and more light emission becomes possible, distribution of that light in an effective manner becomes more difficult.
According to the invention there is provided an illumination system suitable for illuminating an instrument, comprising an information bearing surface; a substantially transparent protective cover; and a first light source coupled to the protective cover such that the protective cover acts as a light guide; wherein the protective cover is arranged to allow light to escape at a selected location to illuminate a portion of the information bearing surface.
The illumination system of the invention is particularly suitable for illumination of vehicle instruments. According to another aspect of the invention there is also provided an instrument suitable for use in a vehicle, comprising an information bearing surface; a substantially transparent protective cover having an interior surface and an exterior surface; and a first light source coupled to the protective cover such that the protective cover acts as a light guide; wherein the protective cover is arranged to allow light to escape at a selected location into the interior of the instrument to illuminate a portion of the information bearing surface.
The information bearing surface may be front lit by reflection of escaped light. Alternatively a surface of the protective cover may comprise the information bearing surface, the information bearing surface may be back lit by reflection of escaped light from an interior surface.
The instrument may be lit using two or more light sources of different colors, and the colors may be combined to provide different controlled variations in color. Therefore in one embodiment the instrument has a plurality of light sources, and in which each light source produces light of a different color from the light produced by each other of light sources.
It is possible to provide a flexible illumination system in which different options are used in different vehicle models, therefore in another embodiment of the invention, the instrument further comprises a second light source, arranged to backlight the information bearing surface, such that the information bearing surface may be illuminated by the first light source, by the second light source or the first light source and the second light source simultaneously.
Advantageously the protective cover has a focussing element on the interior surface to focus escaped light. Furthermore it may also have a chamfered edge around the perimeter of the protective cover.
A pleasing appearance may be achieved by allowing light to escape from the exterior of the instrument, for example, to provide a feature light around the perimeter of the instrument. Accordingly in one embodiment light is arranged to escape from the protective cover to the exterior of the instrument at a selected location.
The invention allows an instrument cluster to be used in which the information bearing surfaces are not coplanar. A problem with instrument clusters in which the information bearing surfaces are not coplanar is that illumination is difficult to achieve without the information surface closest to a light source causing shadowing for an information surface which are further from the light source. Accordingly, in another aspect the invention also provides instrument cluster comprising a plurality of instruments using the illumination system described above arranged such that the information bearing surfaces of a first instrument is other than coplanar with the information bearing surface of a second instrument.